When the movie-title hall of fame is built, surely “Hot Tub Time Machine” will be honored in the same wing as “Snakes on a Plane.” What is “Hot Tub Time Machine” about? It is about a hot tub that is also a time machine. The title gives you permission not to watch it if you think it sounds stupid. Then again, time travel in a hot tub is no dumber than time travel in a DeLorean or a phone booth, and at least “Hot Tub Time Machine” tells you up front.

‘Tis a comedy, of course, not a science-fiction adventure. The closest the film comes to “science” is mentioning that the hot tub became a time machine because someone spilled a can of Russian soda in the controls. As for the time-travel paradoxes, you get the feeling they made a list of all the ones in “Back to the Future” and then came up with ways to make them filthy. Encountering your parents before they conceived you? Normal. Encountering your parents while they’re conceiving you? “Hot Tub Time Machine.”

Adam (John Cusack), Lou (Rob Corddry), and Nick (Craig Robinson), best friends in high school, have grown apart now. Adam’s a sad-sack insurance agent whose girlfriend has just left him. Nick, once a musician, now works for a dog groomer. Lou is a dangerous alcoholic — if this is “The Hangover,” which in many ways it is, he’s Zach Galifianiakis — who lands in the hospital after what may have been a suicide attempt.

This is all the pretense the movie needs for Adam to suggest they take a trip to Kodiak Valley, a ski resort they visited in their youth that holds many good memories. Surely this will lift their spirits and rekindle their friendship. With Adam’s 20-year-old loser nephew, Jacob (Clark Duke), tagging along, the group returns to find the place rundown and seedy now, with a surly one-armed bellhop (Crispin Glover) and a malfunctioning hot tub. And when I say it’s malfunctioning, I mean that when they get in, it sends them back to 1986.

As 17-year-olds, they were at Kodiak Valley this particular 1986 weekend — well, not Jacob; he wasn’t born yet — but there’s no danger that they’ll run into themselves. For some reason Adam, Lou, and Nick actually become their younger selves. That’s what they see when they look in the mirror, and it’s what the people around them see. I assume this is because the filmmakers didn’t want to deal with the complications inherent in characters meeting past versions of themselves. I don’t blame them, though it does violate the rules of pretty much every time-travel story.

The guys remember this weekend well. To avoid screwing up the future, they vow to let everything happen as it originally did. At first, anyway. Then, as with most elements in this scattershot film, they change their minds and decide to make the future better. Lou is going to stand up to that preppie jerk Blaine (Sebastian Stan) this time around, Adam isn’t going to break up with his girlfriend (Lyndsy Fonseca), and Nick’s band is going to give the concert of their careers! Meanwhile, Jacob — who shouldn’t exist in 1986 at all — tries to get straight answers from a mysterious hot tub repairman (Chevy Chase) who only speaks in riddles.

The screenplay was originally written by first-timer Josh Heald, then underwent rewrites by Sean Anders and John Morris (“Sex Drive”). Anders and Morris were supposed to direct the film, too, but were replaced by Steve Pink, an old buddy of Cusack’s who wrote “High Fidelity” and “Grosse Pointe Blank.” (He also directed the under-appreciated 2006 comedy “Accepted.”) All that shuffling isn’t uncommon in Hollywood, but it might account for why “Hot Tub Time Machine” is so unfocused. Sometimes it’s a parody of ’80s comedies (the casting of Cusack and Glover is an inside joke by itself). Sometimes it’s a broad, look-at-how-silly-things-were-in-the-’80s spoof. Sometimes it’s surreal, as when a person in a bear costume makes random appearances; sometimes it’s a cheap gross-out comedy, with poop and barf and bodily fluids; sometimes it’s a straightforward raucous sex comedy, with naked breasts and an aggressive fear of homosexuality.

The plot meanders, feeling less like a story than a series of vignettes. Nick’s non-famous band is performing at the resort — but so is the ’80s mega-group Poison, at the very same time. (And outdoors, in what must be 30-degree weather.) Adam meets a girl (Lizzy Caplan) that he didn’t meet the first time, even though he’s retracing all his same steps. Lou is supposed to fight the preppies at midnight, but it’s a really long time — basically half the movie — before midnight arrives. Jacob is 20 in 2010 but was born nine months after this fateful weekend in 1986, which would make him more like 23. One of the preppy jerks mentions “21 Jump Street,” which didn’t premiere until 1987. (Look, if I can find that out in five seconds, so can the filmmakers.)

None of this sloppiness matters if the movie is funny, of course, and it is, for the most part. Corddry’s intensely focused ticking time bomb of a character is hilarious, and Robinson is quickly becoming one of the funniest deadpan actors in Hollywood. (I overheard someone ask a colleague if anything in the movie is funnier than the part in the trailer when Robinson says the title and then looks at the camera. The answer is no.) Cusack, for his part, mostly plays straight man. I suspect there’s a solid hour of deleted scenes, probably entire deleted subplots, that will appear on the DVD.

This kind of movie is perfect for DVD, actually. It’s not strong enough to warrant multiple theatrical viewings, but at home? Where you can skip the slower parts and go right to the gut-busting sequences? Or just leave the thing on in the background during a party? A hot tub party, perhaps? Now we’re talking.